Interpretive Summary: A diverse number of plant species have developed cohabitation with microorganisms, both fungi and bacteria. Several have also developed cohabitations with viruses. This mode of living together is referred to as symbiotic, and the associated form of microbe within cohabitation is referred to as an endophytic and the association is endophytic. It is assumed that there are benefits derived from such associations, since these infections do not produce diseases. The reason for this is unknown, but since it is symptomless, the association is viewed as distinct and evolutionary important. After several decades of active research on plant endophytes by an international cadre of researchers, we have learned much and have some important hypotheses. The hypothesis of ‘defensive mutualism’ has dominated thinking regarding the biological effects of fungal endophytes on plants, and this idea has helped to shape and inspire many investigations, and we base our discussion on the combined research generated from our laboratories on the basic premise that such interactions have the potential for exploitations, most of which will be helpful in the areas of biotechnology, agriculture, pharmacy, and ecology.

Technical Abstract:
This work in Fungal Ecology is focused on the group of plant symbionts that have been termed collectively ‘microbial endophytes’. Broadly, microbial endophytes are commonly considered to be any of a diverse group of bacteria, cyanobacteria, or fungi that colonize internal tissues of plants. After several decades of active research on plant endophytes by an international cadre of researchers, we have learned much and have some important hypotheses. The hypothesis of ‘defensive mutualism’ has dominated thinking regarding the biological effects of fungal endophytes on plants, and this idea has helped to shape and inspire many investigations. Over years of research on endophytic microbes, we have developed an appreciation that they are diverse taxonomically and inhabit most or all plants. Whether they are partially endophytic or entirely endophytic, they have come to represent a plethora of ‘unknown advantages’ for hosts; and to be potential sources of new secondary metabolites that could serve as new medicines and agrichemicals. In a sense, the vast majority of endophytes are mysterious microbes whose potential values are yet to be discovered. We present in this discussion several aspects of the world of endophytic microorganisms that document their future applications.